It is frequently desirable in electronic systems to have a set of precise reference voltages available. One such electronic system is the flash A/D converter. In a conventional flash A/D converter, 2.sup.n precise reference voltages typically are needed for comparison to an unknown analog input voltage, where n is the number of digital bits of the converter. Conventional circuitry for generating the needed reference voltages comprises 2.sup.n +1 resistors connected in series between two available reference voltages (one of which reference voltages may be ground). The series string of resistors divides the difference in the two available reference voltages into 2.sup.n additional reference voltages. In a 6-bit A/D flash converter, for example, 65 resistors may be connected in series between ground and an available +3.0 V reference voltage to provide 64 additional reference voltages between ground and +3.0 V. For accuracy of the A/D conversion process it is important that the incremental differences in these additional reference voltages be as precise as possible.
To satisfy cost, size, and reliability objectives, where feasible, it is generally advantageous to fabricate electronic circuits such as the flash A/D converter with monolithic integrated circuit processing technology. With present monolithic integrated circuit fabrication technology, though, it is impractical to fabricate a string of resistors wherein the resistance ratios are sufficiently accurate to provide a flash A/D converter having a resolution of more than 9 bits.
In accordance with the foregoing, a need exists for a method and circuitry for generating a set of highly accurate DC reference voltages for use in electronic systems, and especially for use in the implementation of flash A/D converters in monolithic integrated circuits.